During fertilization, the sperm and the egg, each carrying half a set of chromosomes (haploid), combine to form a single cell with a full set of chromosomes (diploid). This zygote then undergoes a series of cell divisions, known as mitosis, which results in the development of an embryo. As the embryo grows and develops, each new cell receives a complete set of chromosomes, ensuring that all the cells in the body have the same genetic information.